There are many applications for which it is necessary to measure the concentration of a component or components in a multi-component fluid system. Foods which are complex multi-component systems often need to have their constituents analyzed. A typical foodstuff for which its components need regular analysis is milk. Milk contains globules of butterfat, as well as somatic cells, proteins, lactose, and mineral salts dispersed as an aqueous emulsion. Various methods for analyzing foodstuffs are used. Spectroscopy, electrical conductivity and classical “wet chemistry” analytical methods are, or have been, employed in the past.
Over the last decade there has been increased interest in the use of ultrasound in characterizing the constituents of foods as well as other multi-component systems. Systems which have been analyzed by ultrasound techniques include solutions, suspensions and emulsions. Ultrasound has major advantages over many other analytical methods because it is nondestructive, rapid, relatively inexpensive, and can be applied to concentrated and/or optically opaque samples.
Ultrasound techniques use high-frequency sound waves, those in the upper kilohertz and megahertz frequency range, which are propagated through the material being tested. Information about the properties of the material is obtained by measuring the interaction between the propagated sound wave and the material.
It is known that the natural resonance frequencies of a liquid-containing acoustic resonator are linearly related to the velocity of ultrasound waves. In general, once a standing wave is obtained, one changes the applied frequency and monitors the wave's amplitude and phase at a detecting transducer as a function of applied frequency. This information facilitates calculation of the velocity and attenuation of the sound waves in the liquid. These parameters are related to, and can provide information about, the characteristics of the liquid, including the concentration of its constituents.
Prior art discusses using cylindrical standing wave ultrasonic resonators to determine the concentration of components in multi-component systems, such as milk, by measuring changes in ultrasonic acoustic properties. However, the methods and apparatuses discussed in prior art have drawbacks. For example, prior art methods require a second resonator to measure a reference sample. They also include measurements of the liquid's acoustic properties at different temperatures-and therefore require waiting for temperature equilibration of the sample when the sample is heated or cooled. In many ultrasound measurements of multi-component systems, flow-through samples can not be measured. Additionally, in prior art systems, ultrasonic methods generally only determine the concentration of small, dissolved species in multi-component systems, that is species present at concentrations greater than 0.01%. The concentration of larger particles, for example somatic cells or microbes in milk which are present at concentrations of less than 0.01%, cannot be determined. Furthermore, prior art systems can determine only a limited number of components typically up to about five components. The number of components is limited because the number of acoustic parameters which is measurable is limited.